1. Field of the Invention
The present invention relates to a resin composition comprising an ethylene-vinyl alcohol copolymer (EVOH) and a multi-layered structure comprising the resin composition. When fabricated into shaped articles, the resin composition adheres little to dies and ensures good long-run workability and good self-purgeability in working lines. The shaped articles fabricated have good appearance with few voids and are yellowed little.
2. Description of the Related Art
EVOH is a useful polymer material having good gas (e.g., oxygen) barrier properties, oil resistance, antistatic properties and mechanical strength, and is widely used for various wrapping and packaging materials such as films, sheets and containers. Such wrapping and packaging materials are generally produced by working polymers in melt. Therefore, the polymers for them are required to have good long-run workability while worked in melt (that is, they can be worked into good products with neither fish eyes nor streaks even in long-run working lines), and their shaped articles are required to have good appearance (that is, they have few gels).
However, since the hydroxyl groups existing in the molecule of EVOH are chemically active in some degree, EVOH is, when melted at high temperatures, often oxidized and crosslinked even in an extrusion machine that contains little oxygen therein. When worked continuously in a long-run working line, EVOH is often thermally degraded to form thermal deposits in the working machine, as its thermal stability is not so good. Such thermal deposits are problematic, as causing fish eyes in the shaped articles of EVOH.
To solve the problem, known are various methods of adding minor acids and/or metal salts to EVOH.
(1) An EVOH composition is disclosed, which contains from 0.0005 to 0.05% by weight (in terms of the metal element) of a salt of a metal of Group 2 of the Periodic Table, from 0.002 to 0.2% by weight of an acid having a pKa of at least 3.5 and a boiling point of not lower than 180° C., and from 0.01 to 0.2% by weight of an acid having a pKa of at least 3.5 and a boiling point of not higher than 120° C., and having a specific fluidity characteristic (JP-A-66262/1989, EP 308,703). They say that the EVOH composition with such additives has improved long-run workability and its shaped articles have good appearance with few fish eyes.
(2) Also disclosed is an EVOH resin composition, which contains from 100 to 5000 ppm, in terms of the free acid, of a hydroxycarboxylic acid and/or its salt, from 50 to 500 ppm, in terms of the metal element, of an alkali metal salt (C), and from 20 to 200 ppm, in terms of the metal element, of an alkaline earth metal salt (B) (JP-A-67898/1998). They say that the composition is, when worked into shaped articles, yellowed little and has good adhesiveness, and that its shaped articles have few fish eyes.
(3) Disclosed is an EVOH composition substantially containing from about 0.01 to about 0.5% by weight of a monovalent or divalent metal salt of at least aliphatic carboxylic acid having from 3 to 9 carbon atoms, and from about 0.05 to about 0.5 parts by weight of at least one hindered phenolic antioxidant (JP-A-227744/1992). They say that the EVOH composition is stable even at high temperatures and is oxidized little to form gel.
(4) Disclosed is an EVOH resin composition containing from 0.001 to 1.0% by weight, in terms of the boron element, of a boron compound (A), from 0.001 to 0.1% by weight of sodium acetate (B), from 0.001 to 0.1% by weight of magnesium acetate, and at most 0.01% by weight of acetic acid (JP-A-60874/1999). They say that the composition ensures good long-run workability when worked in melt, and gives shaped articles having good appearance. Comparative Example 6 given in the publication is an EVOH resin composition containing 0.031% (in terms of the boron element) of boric acid, 0.0093% by weight (in terms of sodium) of sodium acetate, and 0.120% by weight (in terms of magnesium) of magnesium acetate, but they say that the comparative resin composition could not be formed into film since its melt viscosity is too low.
(5) Disclosed is an EVOH resin composition comprising 100 parts by weight of EVOH, at most 0.05 parts by weight of acetic acid, and from 0.001 to 0.02 parts by weight, in terms of the metal element, of magnesium acetate and/or calcium acetate (JP-A-106592/1999, EP 906,924). They say that the resin composition ensures, when worked in melt, long-run workability and is formed into shaped articles having good appearance, and that, when used in producing laminates, the resin composition is odorless and ensures good interlayer adhesiveness.
In the related art disclosures (1) to (5), the EVOH compositions could ensure better long-run workability and their shaped articles could have better appearance than before. However, when they are continuously worked for an extremely long period of time and when a part of them stay in the working machine for a long period of time, they are often crosslinked and thickened and are finally gelled. Therefore, it is desired to develop an EVOH resin which is yellowed little even in high-level long-run working lines and can be shaped into articles having good appearance with few fish eyes.
For preventing EVOH articles from having fish eyes, it is desired not to increase the melt viscosity of the resin composition. However, EVOH compositions of which the melt viscosity is too low are problematic in that they could not be formed into films. Even when such EVOH compositions having a low melt viscosity could be formed into films by specifically controlling the working condition for them, the films will have many voids (small bubbles) or, as the case may be, will have large-size holes, and, as a result, their appearance is extremely bad. This is because the vapor resulting from the decomposition of EVOH being worked forms voids when its amount is not so large, but forms large-size holes when its amount is large. Small voids look like gels, but actually differ from them. Concretely, gels are in the form of small hills rising from the surface of film, but contrary to these, voids are in the form of recesses caving in the surface of film.
When working lines for EVOH are temporarily suspended on weekends, in general, EVOH in the lines is purged with polyethylene and the lines are turned off. However, if a small amount of EVOH has still remained in the turned-off lines, it will be degraded while the lines are cooled or heated, and will form a gel when the lines are again turned on. The gel is problematic in that it worsens the appearance of the films or sheets formed in the thus re-turned-on lines, and a lot of time is taken until the films or sheets formed in the re-turned-on lines can have good appearance with no gel. (The gel formation is caused by the self-purging failure in the working line of EVOH.)
It is therefore desired to obtain a resin composition comprising an ethylene-vinyl alcohol copolymer, which ensures good long-run workability and good self-purgeability in working lines and which is yellowed little and can be shaped into articles having good appearance with neither voids nor holes, and also to obtain multi-layered structures comprising the resin composition.